...that the Performace Level (PL) is reduced with a series connection of safety switches with mechanical contacts?

In order to save costs, safety switches of several safety doors are often connected in series to a safety relay.

However, the diagnostic capability of the faults is greatly reduced with a series connection of door switches with mechanical contacts. This makes it difficult to determine the achievable performance level. This topic is described in EN ISO 14119 in paragraph "8.6 Logic series connection of interlocking devices" and reference is made to the technical report ISO/ TR 24119. In the past, the same degree of diagnostic coverage (DC) was often incorrectly assumed for mechanical safety switches with a series connection and a DC of 99% was specified by the manufacturer.

However, in a series connection the actual DC often shrinks below 60% and the achievable performance level of PLe drops to PLc. For this reason, many machines are unnoticed equipped with an inadequate PL and are therefore not safe. According to ISO/TR, these faults are referred to as fault concealment, but EN ISO 13849-1 requires for Cat. 3 or Cat. 4 that every first fault is detected by the system and that the protective function is not impaired. For this reason, no category 3 can be claimed for these machines and the performance level PLe is not achieved, regardless of whether the DC is above 60%.



Figure 1: All doors are closed, No error in the safety circuit, Motor running


Figure 2: All doors are closed, Error in the safety circuit (cross cirguit), Fault due to safety relay not detected, Motor running


Figure 3: Door 1 opened, Error in the safety circuit, 2-channel error is detected by the safe relay (only one channel switches off), Motor stopped

Figure 4: All doors are closed, Error in the safety circuit, 2-channel error is detected by the safety relay, Motor stopped


Figure 5: Door 2 is opened, Error in the safety circuit, Errors are cleared in the safety relay by opening both channels, Motor stopped

Figure 6: All doors are closed, Error in the safety circuit, But no error detected in the safety relay (error overwritten by opening both channels), Motor running 


The SAFIX 3 safety sensors and the HOLDX R process guard locking have safe OSSD outputs in the output circuit. The use of OSSD outputs changes neither the wiring category nor the diagnostic coverage (DC) according to EN ISO 13849-1. Every single error that occurs is detected in the system and leads to a safe shutdown. Several safety switches up to PLe can be connected in series without any problems. If the safety sensors are cascaded (connected in series), only the PFHD value of the entire circuit must be calculated. For the validation software SISTEMA libraries are available which can be downloaded from the SSP website. 


The built-in EDM function monitors downstream, positively driven contactors. A safety relay is no longer necessary.


Error detection in the system thanks to the built-in OSSD outputs Further information on page 2